fix(bulk-upload): i18n hardcoded strings in BulkDropZone and FileSwitcherStrip

- Add bulk_drop_desc, bulk_select_files, bulk_drop_zone_label, bulk_remove_file
  keys to de/en/es message files
- BulkDropZone: use m.bulk_drop_zone_label(), m.bulk_drop_desc(),
  m.bulk_select_files() — removes all hardcoded German
- FileSwitcherStrip: use m.bulk_remove_file() on × button; move aria-live
  from <ul> to a dedicated visually-hidden region above the strip (screen
  readers now announce changes without coupling the live region to the list)
- Spec: import FileEntry from component instead of re-declaring; use
  data-remove-id selector instead of hardcoded German aria-label

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/document/BulkDropZone.svelte

@@ -12,7 +12,7 @@ let isDragging = $state(false);
 
 <div
 	role="region"
-	aria-label="Dateien ablegen"
+	aria-label={m.bulk_drop_zone_label()}
 	data-testid="bulk-drop-zone"
 	class="flex flex-1 flex-col items-center justify-center p-6"
 	ondragover={(e) => {
@@ -54,17 +54,13 @@ let isDragging = $state(false);
 		<p class="font-serif text-base font-bold text-ink">{m.bulk_drop_hint()}</p>
 
 		<!-- Sub description -->
-		<p class="text-sm leading-relaxed text-ink-2">
-			Für jede Datei wird ein eigenes Dokument erstellt.<br />
-			<strong class="text-ink">Der Titel</strong> wird aus dem Dateinamen vorausgefüllt —
-			<strong class="text-ink">alle anderen Felder</strong> gelten für alle gemeinsam.
-		</p>
+		<p class="text-sm leading-relaxed text-ink-2">{m.bulk_drop_desc()}</p>
 
 		<!-- CTA button -->
 		<label
 			class="flex min-h-[44px] cursor-pointer items-center rounded-sm bg-primary px-6 py-2 text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-90"
 		>
-			Dateien auswählen
+			{m.bulk_select_files()}
 			<input
 				type="file"
 				multiple